Flexible Connections for Flexible Solar Cells

While today’s solar cells are costly and made out of silicon, tomorrow’s may be made out of plastic and be very inexpensive. However, these polymer solar cells must be flexible. That’s a problem for the device’s cathode and anode. These electrical connectors must be transparent, conductive, flexible and compatible with plastics processing — a challenging combination.

On the left is the structure of a polymer solar cell with a flexible anode. The anode consists of the PEDOT PH500 and PEDOT-EL layers, and the device thus constructed has about 80 percent of the power conversion efficiency of a reference polymer solar cell. On the right is a photo of flexible solar cells. Reprinted from Applied Physics Letters.Now a group from Linköping University in Linköping, Sweden, and from Jilin University in Changchun, China, has part of the solution: a polymer anode design that’s simple to prepare and performs well. Fengling Zhang, a team member and Linköping associate professor of biomolecular and organic electronics, said, “This recipe can therefore definitely be viewed as a step closer to commercial applications.”

In the past, polymer solar cell anodes were made with the transparent conductor indium tin oxide. However, the material is incompatible with the processing of plastic because its deposition causes the plastic underneath it to shrink. Thus, researchers have sought another anode material.

A conductive polymer

Recently, a highly conductive polymer known as PEDOT:PSS or PH500 from H.C. Starck of Goslar, Germany, was used to replace ITO in organic light-emitting diodes. The same polymer through high-temperature annealing also has been used as a solar cell anode on glass substrates.

Given that, the group used the material in a flexible anode for polymer solar cells, first trying a single layer of PH500. The performance, recalled Zhang, was good, with a power conversion efficiency of 1.8 percent. The comparable figure for a cell on an ITO-coated glass substrate is about 2.8 percent.

Unfortunately, a key electrical parameter, open circuit voltage, decreased by about 0.16 V from that of a device using ITO on glass as an anode. That change was too great for use in a commercial device and possibly arose from the high conductivity and surface roughness of the PH500 anode.

So the investigators added a second polymer layer, low-conductivity PEDOT-EL, as a buffer between the active layer in the solar cell and the PH500. Previous work indicated that this would likely help, and it was recognized that PEDOT-EL improves anode smoothness.

The resulting bilayer polymer anode had a power conversion efficiency of 2.2 percent, which was about 80 percent that of the reference cells built using ITO on the glass anodes.

These results are promising, particularly for roll-to-roll processing of large-area polymer solar cells. Zhang noted that the next step will be to develop an analogous cathode, which will eliminate the need for any vacuum processing steps in the manufacture of polymer solar cells. Optimization should improve performance further.

However, she added, developing anodes and cathodes is only part of what is needed, noting that currently one of the biggest concerns is the rather limited photocurrent produced by such solar cells.

“This is where improvements are necessary to make polymer photovoltaics a commercially viable technology,” Zhang said.